Doctor of Philosophy / Department of Chemistry / Mark D. Hollingsworth / The ferroelastic urea inclusion compound (UIC) of 2,10-undecanedione/urea exhibits a striking pseudoelastic memory effect. Although pseudoelasticity is possible for UICs containing only 2,10-undecanedione, introduction of a structurally similar guest impurity (2-undecanone) gives rise to rubber-like behavior, a form of pseudoelasticity. This phenomenon depends on both the crystal strain and the concentration of monoketone: above 13-14% 2-undecanone, pseudoelastic behavior is observed reliably, even at strains as high as 2.4%. The dramatic change in ferroelastic behavior over a small range of impurity content indicates that this is a critical threshold phenomenon. Because the impurity concentration has such a dramatic effect on domain switching, it was important to determine the sector-dependent patterns of incorporation of this relaxive impurity. Preliminary HPLC analyses of guest populations suggest that preferential incorporation of monoketone guests occurs between nonequivalent growth sectors, and that these patterns can be rationalized using a symmetry specific growth model. Birefringence mapping and HPLC studies of optically anomalous UICs containing mixtures of 2,9-decanedione and 2-decanone (which possess trigonal metric symmetry) suggest analogous patterns in guest incorporation and/or ordering that can also be rationalized. Although crystals of 2,9-decanedione/urea exhibit no ferroelastic strain at ambient temperature, they exhibit a proper ferroelastic phase transition near -170[degrees]C. It is proposed that differential perfection of domains gives rise to pseudoelasticity in UICs, and that relaxive impurities play an important role in the energetics of this process. Because ultrafast video studies of domain reversion kinetics demonstrate no clear correlation of observed rates with impurity content, it is proposed that the relaxive impurities facilitate spontaneous domain reversion by annealing stressed defect sites that would otherwise lead to irreversible or plastic domain switching. Following earlier work using synchrotron white beam X-ray topography, the driving force for domain reversion is thought to involve the presence of nanoscopic twins whose strain is epitaxially mismatched with neighboring daughter domains. The behavior of these nanoscopic twins was monitored with in-situ X-ray diffraction studies of stressed crystals, and this has led to a more thorough understanding of the role of these nanoscopic twins in the ferroelastic domain switching and rubber-like behavior in this class of materials.

Ce travail a porté sur l'étude des phases métastables de l'alliage à mémoire de forme CuAlBe sous différents types de sollicitations mécaniques et thermiques par diffraction des neutrons. Il a permis de définir un protocole expérimental de caractérisation des transformations des phases métastables caractéristiques de l'effet mémoire de forme et de la superélasticité. Après élaboration par filage à chaud suivi d'une trempe à l'eau, le matériau est entièrement austénitique β1. Sa microstructure est composée de grains de taille relativement importante, de l'ordre de 400 µm. Ce procédé de fabrication génère une texture cristallographique de type fibre partielle <001>. L'étude de la superélasticité lors d'un essai de traction à température ambiante a mis en évidence le comportement pseudoélastique de l'alliage. L'étude de l'évolution des microdéformations a permis de mettre en avant la forte hétérogénéité de comportement du plan (400). Le pic de diffraction de ce plan présente également un fort élargissement dû aux fautes d'empilements qui est directement à relier à la transformation de phase de l'austénite en martensite. La martensite β'1 de structure monoclinique 18R complexe a été affinée à l'aide d'un modèle de type 6M. Ce modèle permet de rendre compte au mieux de la faible périodicité des fautes d'empilement caractéristiques de cette phase métastable à notre échelle d'analyse caractéristique d'un volume de l'ordre du centimètre cube. Après déformation plastique, la texture cristallographique du matériau a fortement évoluée. Le laminage à froid fait disparaitre la fibre partielle <001>. Aux plus forts taux de déformation plastique par laminage à chaud, la fibre <111> apparait. Cette forte déformation affecte également l'orientation des lattes de martensite. D'autre part, les températures des transformations de phases ainsi que l'hystérésis sont modifiées. Cependant, la structure cristallographique de la martensite générée par déformation plastique est identique à celle obtenue par refroidissement pour notre échelle d'observation. L'étude du retour à l'équilibre des phases métastables après recuit à haute température suivi d'une trempe sur un échantillon déformé plastiquement a montré la disparition totale de la martensite et l'apparition des phases stables α et γ2 pour des températures de recuit entre 500°C et 600°C. Au-delà de 600°C, ces deux phases disparaissent au profit de la phase β. On observe alors un fort grossissement du grain. La texture cristallographique est de nouveau caractérisée par la fibre partielle <001>. / This work deals with the study of metastable phases of CuAlBe shape memory alloy under mechanical and thermal solicitations by neutron diffraction. It enables to define an experimental protocol of characterization of metastable phase transformation.The raw material is fully austenitic at room temperature. Its microstructure is composed by huge grain size, close to 400 µm. The crystallographic texture is characterized by a <001> partial fibber.The study of the superelasticity during a tensile test at room temperature demonstrated the pseudoelastic behaviour of this material. The evolution of microdeformations showed the heterogeneous behaviour, especially for the (400) plane in axial direction. The diffraction peak of this plane family also has an important increase of the width. This increase can be linked to the transformation of the austenite into martensite.The crystallographic structure of the monoclinic martensite β'1 has been refined using the 6M model. This model enables to report the relatively low periodicity of stacking faults characterizing the martensitic transformation.After plastic deformation, the crystallographic texture evolved. The <001> partial fibber disappears. For the larger deformation rates, the <111> fibber appears.This large deformation also affects the martensite variant orientation and modifies the temperature of phase transformation.The return into equilibrium of metastable phases after annealing treatments between 500°C and 600°C followed by a quenching at room temperature on a plastically deformed sample has shown the disappearance of martensite and the growth of α and γ2 stable phases. Beyond 600°C, the grains grow largely. The crystallographic texture is characterized by the <001> partial fibber.

Orientador: Paulo Roberto Mei / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-18T15:15:33Z (GMT). No. of bitstreams: 1 Bueno_JulianaCristina_M.pdf: 6612143 bytes, checksum: ff1f20ccef9c00e4455c0b43362c43ce (MD5) Previous issue date: 2005 / Resumo: Neste trabalho foi estudada uma liga inoxidável Fe-Mn-Si-Cr-Ni-Co com efeito de memória de forma (EMF) e os objetivos principais foram otimizar a técnica de coloração por ataque químico (color etching) para identificação e análise das fases presentes na microestrutura, o que permitiu a determinação da dureza da martensita-? e da austenita-? através da técnica de nanoindentação. O desenvolvimento deste processo também permitiu a quantificação das fases e e g por microscopia óptica. A técnica de coloração por ataque químico consiste na utilização de reagentes específicos que resultam em uma microestrutura composta por várias colorações, o que permite identificar fases por microscopia óptica. Os resultados de dureza obtidos por nanoindentação foram de 7,0 GPa para a martensita-? e de 3,0 GPa para a austenita-?. Já para a fração volumétrica da martensita-?, os resultados obtidos por microscopia óptica variaram de 33 a 40 % para amostras no estado deformado. Para amostras de tamanho de grão 123 ?m e 3º ciclo de treinamento, os resultados foram coerentes com os obtidos por difração de raios X de trabalhos anteriores para a mesma liga / Abstract: In this work a Fe-Mn-Si-Cr-Ni-Co stainless alloy with shape memory effect (SME) was studied and the main objectives were to optimize the technique of coloration by chemical attack (color etching) for identification and analysis of the phases in the microstructure, allowing the determination of the ?-martensite and the -austenite hardness by nanoindentation technique. The development of this process also allowed the quantification of the ?- and ?- phases by optical microscopy. The technique of coloration by chemical attack consists in the use of specific reagents to identify phases by color using optic microscopy. The hardness obtained was 7.0 GPa for ?-martensite and 3.0 GPa for ?-austenite. The volume fraction of the ?-martensite, measured with optical microscopy varied from 33 to 40 % for samples in the deformed state. For samples with grain size of 123 ?m and after the 3º training cycle, the results were coherent with the obtained by X ray diffraction of previous works for the same alloy / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

No description available.

The martensite and magnetic transformations in Ni-Fe-Ga ferromagnetic shape memory alloys are very sensitive to both alloy chemistry and thermal history. A series of Ni-Fe-Ga alloys near the prototype Heusler composition (X2YZ) were fabricated and homogenized at 1423 °K, and a Ni₅₃Fe₁₉Ga₂₈ alloy was subsequently annealed at various temperatures below and above the B2/L21 ordering temperature. Calorimetry and magnetometry were employed to measure the martensite transformation temperatures and Curie temperatures. Compositional variations of only a few atomic percent result in martensite start temperatures and Curie temperatures that differ by about 230 °K degrees and 35 °K degrees, respectively. Various one-hour anneals of the Ni₅₃Fe₁₉Ga₂₈ alloy shift the martensite start temperature and the Curie temperature by almost 70 °K degrees. Transmission electron microscopy investigations were conducted on the annealed Ni₅₃Fe₁₉Ga₂₈ alloy. The considerable variations in the martensite and magnetic transformations in these alloys are discussed in terms of microstructural differences resulting from alloy chemistry and heat treatments. The phase-field method has been successfully employed during the past ten years to simulate a wide variety of microstructural evolution in materials. Phase-field computational models describe the microstructure of a material by using a set of field variables whose evolution is governed by thermodynamic functionals and kinetic continuum equations. A two dimensional phase-field model that demonstrates the ferromagnetic shape memory effect in Ni2MnGa is presented. Free energy functionals are based on the phase-field microelasticity and micromagnetic theories; they account for energy contributions from martensite variant boundaries, elastic strain, applied stress, magnetocrystalline anisotropy, magnetic domain walls, magnetostatic potential, and applied magnetic fields. The time-dependent Ginzburg-Landau and Landau-Lifshitz kinetic continuum equations are employed to track the microstructural and magnetic responses in ferromagnetic shape memory alloys to applied stress and magnetic fields. The model results show expected microstructural responses to these applied fields and could be potentially utilized to generate quantitative predictions of the ferromagnetic shape memory effect in these alloys. / Ph. D.

Shape Memory Alloys (SMA’s) are intermetallic materials (chemical compounds of two or more elements) that are able to sustain a residual deformation after the application of a large stress, but they “remember” the original shape to which they creep back, without the application of any external force, after they are heated above a certain critical temperature. A general one-dimensional dynamic mathematical model is presented which accounts for thermal coupling, time-dependent distributed and boundary inputs and internal variables. Well-posedness is obtained using an abstract formulation in an appropriate Hilbert space and explicit decay rates for the associated linear semigroup are derived. Numerical experiments using finite-dimensional approximations are performed for the case in which the thermodynamic potential is given in the Landau-Devonshire form. The sensitivity of the solutions with respect to the model parameters is studied. Finally, an alternative approach to the stress-strain laws is presented which is able to capture the dependence on the strain history. / Ph. D.

In this thesis, the thermomechanical approach with internal variables has been thoroughly analyzed. This approach is based on the combination of thermodynamic principles and continuum mechanics. Therefore it reflects the physical essence of constitutive behavior of materials. Based on this approach, a one-dimensional constitutive model for the two-way shape memory effect and a one-dimensional constitutive model for piezoceramics have been developed, respectively. In modeling the two-way shape memory effect, a residual stress σ_re is introduced as a controlling parameter for the two-way shape memory effect. A further refinement of the transformation kinetics expression for two-way shape memory is derived. It is demonstrated that the material parameters required by this model can be calculated or measured using a standard materials testing apparatus. A numerical study is conducted and the effectiveness of this model is verified. In the constitutive modeling of piezoceramics, a new internal state variable is introduced to relate the macroscopic behavior of a piezoceramic with its micro-properties. A phenomenological formulation of polarization reversal is proposed, and then a fully-coupled thermo-electro-mechanical model is developed. It is shown that the theory developed can describe the electromechanical behavior of piezoceramics well. / Master of Science

Les Alliages à Mémoire de Forme (AMF) sont des matériaux actifs ayant des propriétés mécaniques spectaculaires comparées aux autres métaux : effets mémoire simple et double sens, pseudo-élasticité et amortissement. Les propriétés des AMF ont pour origine physique une transformation austénite – martensite pilotée par la température et le niveau de contrainte dans le matériau. Les phases austénite (A) et martensite (M) sont présentes respectivement à haute température et à base température. L’effet mémoire, quant à lui, réside dans la capacité du matériau à retrouver la forme austénitique initiale par élévation de température, après avoir été déformé de manière permanente à l’état martensitique à basse température. Le comportement mécanique des structures en béton est gouverné par le processus d’endommagement du matériau. Ce processus peut être retardé en appliquant un chargement uni ou multi-axial de compression, dans le but de contrer les contraintes locales de traction auxquelles le béton est peu résistant. Cette thèse porte sur l’utilisation d’alliages à mémoire de forme (AMF) pour la création d’états de précontrainte dans des composants en béton. Le travail repose sur deux approches : expérimentation et modélisation. Dans la première partie, des essais préliminaires concernent l’étude du comportement thermomécanique de l’AMF en Ni-Ti. Cette réponse complexe est étudiée de manière séparée à l’aide d’une machine de traction – compression uni-axiale couplée à des moyens de chauffage et de refroidissement. Ensuite, des fils d’AMF sont utilisés pour la création de précontraintes dans des poutrelles et de confinements dans des cylindres en béton. Les fils sont étirés à l’état martensitique avant d’être fixés à leurs extrémités sur des éprouvettes en béton. L’activation thermique de l’effet mémoire provoque la mise en contrainte du béton. Et puis, des essais d’écrasement des cylindres sont réalisés pour estimer l’amélioration des performances du béton confiné à l’aide de fils d’AMF. Les résultats montrent que l’effet de confinement permet d’améliorer fortement la performance mécanique en compression du béton. Dans la deuxième partie, un modèle thermomécanique est élaboré pour l’analyse du comportement de fils d’AMF sollicités en traction-compression alternée uni-axiale. Une procédure de calcul numérique pas-à-pas est développé pour la simulation du comportement de fils en AMF pour l’ensemble de la procédure de création d’effet de précontrainte. Cette simulation donne une description fine des mécanismes au sein du fil au cours des essais sur des composants en béton-AMF. L’interaction complexe entre le béton et l’AMF est précisément analysée grâce à l’utilisation du modèle thermomécanique de l’AMF. Enfin, les études de cette thèse confirment une possibilité du champ d’application des AMF dans la thématique du renforcement préventif des structures en béton. / Shape memory alloys (SMAs) are active materials that exhibit special properties such as pseudoelasticity and memory effect. These properties are resulting from austenite vs. martensite reversible transformations governed by temperature and mechanical stress states. The austenite phase (A) and the martensite phase (M) are present respectively at high and low temperature. The shape memory effect is the ability of the material to retain a deformation gained in the martensite phase, i.e. at low temperature, and then to recover its initial shape when it returns to the austenite phase upon temperature increase. The mechanical behaviour of structural concrete is governed by a process of damage. The damaging process can be delayed by applying a uni- or multiaxial compression in order to counterbalance local tensile stresses in the material. The present thesis deals with the use of shape memory alloys (SMAs) to create prestress states in concrete components. The work is based on two approaches: experimental and modelling. In the first part, preliminary tests concern the studies of the thermomechanical behaviour of Ni-Ti SMA. This complex response is studied singly by means of a MTS uniaxial testing machine and heating-cooling systems. Then, SMA wires are used to create prestress states in small-scale concrete beams as well as confinement states in concrete cylinders. They were given a prestrain in a martensitic state before being firmly fixed on concrete components. Thermal activation of the memory effect in the SMA wires caused their tensioning, which resulted in reaction in the creation of stresses in the concrete. Moreover, crush tests of concrete cylinders are performed in order to estimate the improvement of the mechanical performance of concrete confined by means of SMA wires. The test results show that the confinement effect can improve strongly the mechanical performance of concrete. In the second part, a thermomechanical model is performed to analyze the behaviour of SMA with an extension to allow for uniaxial traction-compression. A steps by steps process of the numerical simulation is developed for SMAs during the process of prestress creation. This simulation gives a detailed description of the mechanisms of SMA wires which lead to the process of experimental studies on the SMAs-concrete components. A complex interaction between the concrete and the SMAs is evidenced by means of the thermomechanical model of SMAs. Finally, studies presented in the present thesis confirm the possibility to use SMA as preventive reinforcement for application to civil engineering structures.

Les mécanismes de formation et dissociation d’hydrates de cyclopentane (CP), qui forment á pression ambiante et á des températures entre 0ºC et 7ºC, ont été observés dans/sur/proche des gouttes d’eau immergées dans du CP á des échelles qui vont du micron jusqu’au millimètre. Plusieurs techniques d’observation ont été utilisées, telles que la macrophotographie et la microscopie optique en champ clair, par contraste interférentiel différentiel (CID), par fluorescence et par réflectance confocale. Des substrats hydrophiles et hydrophobes ont été utilisés. Dans une première série d’expériences, un procédé millifluidique simple a été mis au point. Il permet de générer, stocker et surveiller simultanément une centaine de gouttelettes de même volume (de l’ordre de μl), régulièrement espacées. Elles sont séparées par la phase ‘invité’ (CP) dans un tuyau en polymère fluoré (PFA) transparent. Chacune d’elles se comporte comme un réacteur indépendant. Une vision sur l’effet mémoire est obtenue en menant des mesures statistiques sur la nucléation des hydrates quand les gouttes d’eau sont refroidies au-dessous de 7°C. Cette méthode permet aussi de visualiser des événements dans des gouttes individuelles, tels que la naissance et la croissance de l’hydrate (surtout lorsqu’un additive tel qu’un inhibiteur est rajouté dans l’eau), ainsi que la formation d’une émulsion de CP dans l’eau pendant la dissociation de l’hydrate. Dans une deuxième série d’expériences, une seule goutte d’eau est posée ou pendue d’un substrat en verre et immergée dans du CP. Elle est observée par microscopie sous des séquences différentes de refroidissement – échauffement. Il a été observé que la cristallisation d’hydrates dépend fortement du sous-refroidissement. Deux nouveaux phénomènes ont été observés:(i) la propagation d’un « halo » d’hydrate le long de l’interface verre/CP depuis la ligne de contact de la goutte d’eau.(ii) cristallisation de l’hydrate dans une émulsion 2D de CP dans l’eau.Les deux types d’outils développés dans cette thèse ouvrent des nouvelles perspectives pour élucider les mécanismes de formation et dissociation d’hydrates en présence d’additives (promoteurs et inhibiteurs) et en présence d’un substrat minéral. Les applications comprennent les hydrates dans des environnements sédimentaires, séparation de gaz, etc. / The mechanisms of formation and dissociation of cyclopentane (CP) hydrates, which form at ambient pressure and temperatures between 0°C and 7°C, have been observed in/on/near water drops immersed in CP at scales ranging from a few nanometers to the millimeter by a variety of techniques including macrophotography and optical microscopy under various modes: bright field, differential interference contrast (DIC), fluorescence and confocal reflectance. The substrates used are either hydrophobic or hydrophilic. In a first series of experiments, a simple millifluidic method is implemented. It allows to generate, store and monitor at the same time almost a hundred of regularly-spaced water droplets of equal volume (in the µl range) separated by the guest (CP) phase in a transparent fluorinated polymeric (PFA) (hydrophobic) tubing, each droplet behaving as an independent reactor for hydrate crystallization. Insights into the ‘memory effect’ are gained by measuring the statistics of hydrate nucleation events in these reactors when chilling below 7°C the water drops. The method also allows the visualization of single-drop events such as hydrate birth and growth, and the formation of a CP-in-water emulsion upon hydrate melting, especially when an additive such as an inhibitor is added to the water. In a second series of experiments, a single water droplet in CP, either sitting or hanging from a glass substrate, is observed by microscopy under various cooling and heating sequences. Hydrate crystallization (nucleation and growth) is observed to strongly depend on subcooling at the water drop/CP interface. Two novel phenomena are visualized in detail:(i) the propagation, from the contact line of the water drop, of a hydrate halo along the glass/CP interface. (ii) hydrate crystallization in a two-dimensional CP-in-water emulsion.The two types of tools developed in this thesis open new perspectives for elucidating the mechanisms of hydrate formation and dissociation in presence of additives (promoters and inhibitors) and in the presence of a mineral substrate. Applications include hydrates in sedimentary environments, flow assurance, gas separation, etc.

Recent proposals have attributed a key role to novelty in the formation of new episodic memories. These proposals are based on evidence of enhanced memory and greater metabolic activity in the hippocampus in response to novel relative to familiar materials. However, such novelty effects are incongruous with long-standing observations that familiar items and lists are associated with better memory than novel ones. In four experiments, I explored possible reasons for this apparent discrepancy. In Experiment 1, I directly tested whether previously observed novelty effects were the result of novelty, discrimination demands, or both. I used linguistic materials (proverbs) to replicate the novelty effect but found it occurred only when familiar items were subject to source confusion. In Experiment 2, to examine better how novelty influences episodic memory, I used experimentally familiar, pre-experimentally familiar, and novel proverbs in a paradigm designed to overcome discrimination demand confounds. Memory was better for both types of familiar proverbs. These cognitive results indicate that familiarity, not novelty, leads to better episodic memory for studied items, regardless of whether familiarity is experimentally induced or based on prior knowledge. I also conducted two fMRI experiments to evaluate the neural correlates of the encoding of novel and familiar forms of information. In Experiment 3, I compared the neural encoding correlates of source memory for novel and familiar visual scenes using fMRI. Replicating previous neuroimaging studies, I observed an anterior novelty-sensitive region of the hippocampus specialized in novelty encoding. Unlike past studies, I also probed for familiarity-encoding regions and identified such regions in the posterior hippocampus. I replicated this pattern in Experiment 4 using proverbs as stimuli. As in Experiment 2, I found the effect held whether familiarity was based on prior knowledge or experimental induction. In both fMRI experiments, anterior and posterior hippocampal regions were functionally connected with different large-scale networks, helping to explain local variation in hippocampal functional specialization in terms of different neural contexts. Together, these experiments show that stimulus familiarity enhances episodic memory for materials, and that novelty is processed differently, not preferentially, in the hippocampus. A new model of hippocampal novelty processing is proposed.